This invention relates to cooling a mass by immersion in a cryogenic liquid, e.g. helium cooled superconducting magnet assemblies suitable for magnetic resonance imaging (hereinafter called "MRI"), and more particularly to an improved and simplified means for the precooling of the mass in order to conserve cryogenic liquid coolant, and avoid introduction of contaminants into the mass being cooled.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by immersion in a liquid cryogen, e.g. liquid helium contained in a cryostat or pressure vessel. The extreme cold ensures that the magnet coils are maintained in superconducting operation, such that when a power source is initially connected to the magnet coils (for a period, for example, of one hour) to introduce a current flow through the coils, the current will continue to flow through the coils even after power is removed due to the absence of electrical resistance in the coils, thereby maintaining a strong magnetic field. Superconducting magnet assemblies find wide application in the field of MRI.
While the use of liquid helium to provide cryogenic temperatures is widely practiced and is satisfactory for MRI operation the provision of a steady supply of liquid helium to MRI installations all over the world has proved to be difficult and costly. As a result, considerable research and development efforts have been directed at minimizing the amount of boiling cryogen such as liquid helium, required to cool the mass initially, and to maintain it's low temperature during continued service.
One method of minimizing the use of and assisting, liquid helium cooling is to utilize an initial auxiliary cooling medium such as flowing liquid nitrogen through the magnet to obtain an initial cool temperature, such as 80-90 K., and then purging the nitrogen (to avoid nitrogen contamination which can cause superconducting magnet instability) before commencing the final cooling by liquid helium cooling to the superconducting temperature.
The purging of the liquid nitrogen is accomplished by flowing pure, warm helium gas through the magnet. This is followed by the introduction of the liquid helium into the magnet to further cool the magnet to the superconducting temperatures (such as 4.degree. K.).
However, the initial introduction of cold liquid nitrogen into the magnet for precooling shocks the magnet, due to strains caused by uncontrolled rapid cooling, and can affect the purity of the helium. For example if the nitrogen is not completely purged from the cryostat or containment vessel prior to filling with liquid helium, helium purification equipment must be able to separate the nitrogen thus making the recovery process more difficult which in turn can decrease the amount of helium which can be recovered.
In addition, the sequential introduction of liquid nitrogen into a cryostat or containment vessel holding the superconducting magnet followed by purging prior to filling the vessel with liquid helium is time consuming. The helium gas purge also warms the magnet from liquid nitrogen temperature, around 80.degree. K., to higher temperatures such as 110.degree. K., requiring more liquid helium and time to cool the magnet to superconducting temperatures. Also, subsequent cooling of the magnet to superconducting temperature could result in nitrogen ice forming within the magnet which could destabilize superconducting operation of the magnet resulting in possible "quenching", a sudden discontinuance of superconducting operation with rapid helium gas boiloff and generation of high pressure within the magnet.
It is accordingly desirable to avoid contamination of the superconducting magnet by liquid nitrogen, to minimize the helium required to obtain superconducting operation, and to reduce the overall time required for the magnet to be cooled to superconducting temperatures.